The obvious one: you could be completely bereft of mana. Maybe you were mana hosed by your cards in MtG or you freaked out and accidentally blew all your mana on the first creep wave in LoL. Pro packing tip: always bring more mana than you think you're going to need. Like charging cables, you'll be glad you have it at the end of the day.

Or you could take it as meaning whatever you just did is PURELY powered by mana. This fireball? Straight out of mana, monkey mofos. Boom.